1. Field of the Invention
The present invention relates to a method, system and program for reducing toner usage in print output.
2. Description of the Related Art
To print an image, a print engine processor, referred to herein as a raster image processor, converts the image in a page description language or vector graphics format to a bit mapped image indicating a value to print at each pixel of the image. Each bit representing a pixel that is “on” is converted to an electronic pulse. The electronic pulses generated from the raster pel data at which to deposit toner turns the laser beam on to positively charge the surface of a rotating drum, which is an organic photo-conducting cartridge (OPC), that has a coating capable of holding an electrostatic charge. The laser beam turns on and off to beam charges at pixel areas on a scan line across the drum that will ultimately represent the output image. After the laser beam charges all pels on the scan line indicated in the raster data, the drum rotates so the laser beam can place charges on the next scan line. The drum with the electrostatic positive charges then passes over negatively charged toner. The negatively charged toner is then attracted to the positive charged areas of the drum that form the image. The paper, which is negatively charged, passes over the roller drum and attracts the toner as the areas of the roller drum with the toner are positively charged to transfer the toner forming the image from the roller drum to the paper.
Many laser printers may filter the bit map images using a look-up table to alter the pulses generated for each pixel to accomplish a certain filtering result. For instance, filters can be used to provide an economy mode where toner is reduced, remove jagged edges, improve print quality enhancement or reduce the density of images. Typically, the laser printer will gather an area of data and replace either one or all the pulse values for the pixels based on the gathered area of pixel data matching a value in the look-up table. Such look-up tables modify the pixel output by altering the pulse normally used for an “on” pixel value with a pulse width modulator to shorten the pulse width to reduce the electric charge the laser beam places on the roller. Reducing the pulse width reduces the charged are for the pel on the roller and, hence, reduces the amount of toner attracted to the roller for that pel, thus reducing the amount of toner used to represent the pel.
There are two prior art techniques for reducing the amount of toner in print output. One technique, referred to as sub-pulse width modulation, reduces the laser current applied to each pel area on the roller to reduce the area of the electrical charge applied to the pel position on the roller, thereby attracting less toner. This technique requires that the laser be constantly switched on and off within each pel to place the sub-pel charge in a portion of the pel on the roller. This process of switching the laser on and off substantially increases electromagnetic radiation (EMI). To ensure that such printers that reduce tone by switching the laser on and off within each pixel comply with established EMI rules in various countries, electromagnetic patches are added to the printer to absorb the EMI radiation. The addition of these electromagnetic patches or other fixes to avoid the EMI radiation pollution significantly increases the manufacturing cost of the printer to a point where the use of sub-pulse width modulation to reduce toner usage in all black areas is not feasible.
A second approach to reduce toner usage is to apply a single symmetrical screen pattern, e.g., a checkerboard, over the total image to subtract pels from the image. Thus, the entire black area of an image is replaced with a checker board pattern to reduce in half the number of pels to which toner is attracted. The problem with this approach is that because data is removed without any consideration to the image structure, it is possible that the algorithm would delete significant portions of the image, such as edge pels that form the outline of the image. This reduces the edge resolution and quality of the image resulting in a “washed-out” appearance.
For the above reasons, there is a need in the art for an improved technique to reduce toner in an image in a manner that does not unduly degrade the quality of the image and to reduce electronic emissions.